1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer, facsimile, and a complex machine of them, and particularly to an image forming apparatus adapted such that a two-component developer, which charges a nonmagnetic toner using a magnetic carrier, is utilized to form a magnetic brush, the magnetic brush forms a toner thin layer on a developing roller, and the toner of the toner thin layer flies on an electrostatic latent image to develop the latent image.
2. Description of Related Art
Conventionally, in an electrographic image forming apparatus, a one-component developing method and a two-component developing method have been known as a developing method using a dry toner.
In the one-component developing method, since no carrier is contained, an electrostatic latent image of a photoreceptor is not disturbed by the magnetic brush formed of the carrier and the toner, which is suitable for enhancing image quality. However, in the one-component developing method, it is difficult to stably maintain a charge amount of the toner. Moreover, in the case of color toner, since permeability is required, the toner needs to be a nonmagnetic toner. Thus, in many full-color image forming apparatuses, the two-component developing method, which uses a carrier as a medium charging and carrying the toner, is employed.
As an image forming method using the two-component developing method, an image forming method by a so-called touchdown development (also referred to as hybrid development) has been known in which a toner thin layer is formed on the toner holding body by the magnetic brush formed on a developer holding body that holds a two-component developer, and the toner thin layer on the toner holding body allows an electrostatic latent image on a electrostatic latent image holding body to be developed and visualized. However, in this developing method, the two-component developing method and the one-component developing method are combined, thereby posing a problem in that there is a difference between an appropriate charge amount of the toner when the electrostatic latent image is developed and an appropriate charge amount of the toner when the toner thin layer is formed. Thus, in some cases, a small amount of the toner in the toner thin layer causes an unfavorable image density, or a problem of development ghost and the like because of poor peeling of the toner thin layer that did not contribute to the development is posed.
Therefore, in Patent Document 1 (Japanese Patent Application Laid-Open No. 2003-280357), there is provided means for applying an AC bias with a duty ratio of 10 to 50% on which a DC bias is superimposed to the developing roll to prevent the occurrence of development ghost and selective development. However, as described above, since the optimal toner charge amount differs between at the time of thin layer formation and at the time of development, the above-described means may be insufficient as the measures.
Meanwhile, one of causes that pose the above-described problems is influence of scattered toner. With scatter of the toner, a developing apparatus is a major site of emergence, such as the scatter when the toner is stirred inside of a housing, and the scatter in the vicinity of the magnetic roller. The scattered toner occurring in the developing apparatus scatters to the inside of an electrophotographic apparatus in which the electrostatic latent image holding body, an optical system apparatus, a charging apparatus, a transfer apparatus and the like are arranged, thereby resulting in various poor images, improper operations and the like including the above-described problems.
In order to solve these problems, Patent Document 2 (Japanese Patent Application Laid-Open No. H8-137256) proposes that a scatter preventing member that is rotatably provided away from, and opposed to the electrostatic latent image holding member, and causes the developer scattered when the developer is supplied to adhere to a surface thereof to prevent scatter to the outside, and scraping means for scraping the developer adhering to the scatter preventing member are used to prevent the toner scatter.
Patent Document 3 (Japanese Patent Application Laid-Open No. 2005-242194) proposes a method in which in the two-component developing method, a collecting roller is provided in a housing opening portion of a developing apparatus to collect scattered toner, and the toner is peeled from the collecting roller to return the same to the developing apparatus.
In Patent Document 4 (Japanese Patent Application Laid-Open No. H11-174792), there is provided an image forming apparatus having control means for changing a bias to be applied to the collecting roller in accordance with an formed image ratio, and if the image ratio is less than 5%, the control means sets a bias obtained by superimposing a DC voltage of the same polarity as normal charge polarity of the developer on an AC voltage as the bias applied to the collecting roller, if it is 5 to 15%, sets only an AC bias, and if it exceeds 15%, sets a bias obtained by superimposing a DC voltage of reverse polarity of the normal charge polarity of the developer on an AC voltage.
However, since the toner scatter amount varies in accordance with the toner replenishment amount, in the method by Patent Document 2, at all times, only a certain amount can be collected. Particularly in the touchdown developing method, the scattered toner may affect the toner thin layer on the developing roller, thereby causing image disturbance, and thus, the toner scatter needs to be further suppressed.
Moreover, in the touchdown developing method, since the toner density of the two-component developer is high, and the fluidity of the developer is unsatisfactory, when the magnetic brush is collected in a two-component developer containing part, the developer is sequentially collected and pressed and compressed, so that air has nowhere to go and the toner is discharged outside together with the air, which easily causes the toner scatter.
Furthermore, in the touchdown method, since when the formed image ratio is low, the toner remains on a sleeve of the developing roller, in the method of Patent Document 2, conversely, the toner on the developing roller may be charged up, thereby increasing the toner scatter amount.
In the touchdown developing method, in the case where a printing ratio (image ratio) is low, for example, when it is less than 5%, much undeveloped toner remains on the developing roller, which is collected by the two-component developer composed of at least the carrier and the toner held on the magnetic roller, and is collected into the two-component developer containing part inside of the developing apparatus together with the two-component developer on the magnetic roller. Accordingly, since the amount of the collected toner is large and thus the toner amount in the collected two-component developer is also large, the toner scatter easily occurs when the two-component developer on the magnetic roller is collected into the two-component developer containing part.
In Patent Document 3, when the toner is peeled from the collecting roller, which collected the scattered toner, to return the same to the developing apparatus, a channel to return the toner through needs to be separately provided, which disadvantageously makes the apparatus larger.
In Patent Document 4, even if the above-described constitution is employed, the toner scatter may not be able to be suppressed. Particularly, in a high-speed machine employing the touchdown developing method and having a drum linear velocity of 180 mm/sec or more, the system in which in order to suppress deterioration of the developer, the total amount of the developer is increased may easily cause the toner scatter. The drum linear velocity of 180 mm/sec corresponds to about 40 sheets/min, that of 250 mm/sec corresponds to about 50 sheets/min, and that of 340 mm/sec corresponds to about 60 sheets/min in A4 size in the lateral direction.
Moreover, in the touchdown developing method, means for stabilizing a toner layer thickness on the toner holding body in order to stabilize developability from the toner holding body onto a photoreceptor drum has been known. This compensates for charge characteristics of the toner changed by the deterioration in the two-component developer and the like by varying a potential difference ΔV between the toner holding body and the two-component developer holding body, and stably keeps the toner thin layer thickness on the toner holding body. However, this is control in which the potential difference ΔV is varied in accordance with the toner charge amount in the two-component developer, and if the toner charge amount is decreased, the toner layer thickness on the toner holding body becomes large, and thus, the potential difference ΔV needs to be reduced for returning the toner layer thickness on the toner holding body to the original thickness. However, since this varies the potential difference ΔV in a direction in which it is hard for the toner to move from the magnetic brush formed on the magnetic roller to the toner holding body, the two-component developer is collected into the two-component developer containing part while leaving much toner that does not contribute to the toner thin layer formation in the magnetic brush on the magnetic roller, thereby resulting in a state where the toner scatter more easily occurs. That is, since the difference in toner charge amount makes the potential difference ΔV differ, and in turn this difference in potential difference ΔV makes the scatter toner amount differ, there is a problem in that in the case where collecting power of the collecting roller is set to be constant, the toner scatter cannot be suppressed.